Image light guide with expanded light distribution overlapping gratings

ABSTRACT

An image light guide for conveying a virtual image has a waveguide that conveys image-bearing light, formed as a flat plate having an in-coupling diffractive optic with a first grating vector diffracting an image-bearing light beam into the waveguide and directing diffracted light. An out-coupling diffractive optic is formed as a plurality of overlapping diffraction gratings including a first grating pattern having first grating vector k1 and a second grating pattern having a second grating vector k2 for expanding and ejecting the expanded image bearing beams from the waveguide into an expanded eyebox within which the virtual image can be seen.

TECHNICAL FIELD

This invention generally relates to electronic displays and moreparticularly relates to displays that use image light guides withdiffractive optics to convey image-bearing light to a viewer.

BACKGROUND OF THE INVENTION

Head-Mounted Displays (HMDs) are being developed for a range of diverseuses, including military, commercial, industrial, fire-fighting, andentertainment applications. For many of these applications, there isvalue in forming a virtual image that can be visually superimposed overthe real-world image that lies in the field of view of the HMD user.Optical image light guides convey image-bearing light to a viewer in anarrow space for directing the virtual image to the viewer's pupil andenabling this superposition function.

In such conventional image light guides, collimated, relativelyangularly encoded light beams from an image source are coupled into aplanar waveguide by an input coupling such as an in-coupling diffractiveoptic, which can be mounted or formed on a surface of the planarwaveguide or buried within the waveguide. Such diffractive optics can beformed as diffraction gratings, holographic optical elements (HOEs) orin other known ways. For example, the diffraction grating can be formedby surface relief. After propagating along the waveguide, the diffractedlight can be directed back out of the waveguide by a similar outputgrating, which can be arranged to provide pupil expansion along onedimension of the virtual image. In addition, a turning grating can bepositioned along the waveguide between the input and output gratings toprovide pupil expansion in an orthogonal dimension of the virtual image.The image-bearing light output from the waveguide provides an expandedeyebox for the viewer.

Although conventional image light guide arrangements have providedsignificant reduction in bulk, weight, and overall cost of near-eyedisplay optics, further improvements are needed. In some instances, thesize of the eyebox is constrained, forcing HMD designs to limittolerances for movement and device placement. Light can often beunevenly distributed over the visual field, leading to hot spots, suchas higher levels of light within the center of the field and lower lightlevels within the field periphery. Beam management functions within thewaveguide including beam expansion and light distribution functions canincrease the size of waveguides as well as their manufacturing cost andcomplexity.

SUMMARY OF INVENTION

Embodiments feature compact designs of image light guides for carryingout various functions including pupil expansion, light distribution, andbeam management. For example, instead of expanding the two orthogonaldimensions of the image-bearing beams in separate stages requiringdifferent areas of the light guides to be dedicated to the orthogonalexpansions, embodiments add capabilities to out-coupling diffractiveoptics for expanding both dimensions of the image-bearing beams whileejecting the expanded image-bearing beams in overlapping positionswithin an eyebox. Additional design parameters can be exploited forinfluencing light distributions within and between the image-bearinglight beams. The added functionalities of the out-coupling diffractiveoptic enable more compact designs and the ability to maintain desiredangular relationships among the image-bearing beams for presentingwell-corrected virtual images within the expanded eyeboxes.

An image light guide in accordance with one or more embodiments includesa waveguide having a transmissive substrate with front and back surfacesfor propagating image-bearing light beams along a length of thewaveguide. An in-coupling diffractive optic formed along the waveguidediffracts the image-bearing light beams from an image source into thewaveguide for propagating the image-bearing light beams along the lengthof the waveguide in an angularly encoded form. An out-couplingdiffractive optic spaced apart from the in-coupling diffractive opticalong the length of the waveguide diffracts the image-bearing lightbeams from the waveguide in an angularly decoded form toward an eyeboxfor viewing the image from the image source.

The out-coupling diffractive optic includes a plurality of gratingpatterns in different orientations that overlap along the length of thewaveguide. The overlap can occur within the same plane of the waveguideor within different generally parallel planes. One or more of thegrating patterns is arranged in a primary orientation for successivelydiffracting first portions of each of the image-bearing light beams bothin the angularly decoded form toward the eyebox and in the encoded formfor further propagation in an original nominal direction along thewaveguide. A nonzero order of diffraction matching a similar nonzeroorder of diffraction of the in-coupling diffractive optic is generallyresponsible for diffracting the successive first portions out of thewaveguide in the angularly decoded form, and a zero order of diffractionis generally responsible for further propagating other of the firstportions in the original nominal direction for supporting successiveencounters with the overlapping grating patterns. One or more other ofthe grating patterns is arranged in one or more non-primary orientationsfor diffracting second portions of each of the image-bearing light beamsout of the original nominal direction of propagation along the waveguideand for further diffracting the second portions of each of theimage-bearing light beams back toward the original nominal direction ofpropagation along the waveguide in positions that are relatively offsetin a direction normal to the original nominal direction of propagation.The combination of nonzero-order and zero-order diffractions of thesecond portions provide for successively displacing the second portionsin a direction normal to the original nominal direction of propagation.

The first portions of each of the image-bearing light beams arepreferably relatively displaced in the original nominal direction ofpropagation along the waveguide for expanding a first dimension of anarea of overlap among the image-bearing light beams within the eyebox.The second portions of each of the image-bearing light beams arepreferably relatively displaced in the direction normal to the originalnominal direction of propagation along the waveguide for expanding asecond dimension of the area of overlap among the image-bearing lightbeams within the eyebox. The so-expanded eyebox provides an expandedpupil within which the intended virtual image can be seen.

The one or more of the grating patterns arranged in a primaryorientation can include a first grating pattern having a first gratingvector, and the one or more other of the grating patterns arranged inone or more of the non-primary orientations can include a second gratingpattern having a second grating vector. The second grating vector(considered as an undirected line segment) is preferably inclined at anangle of at least 30 degrees to the first grating vector (considered asan undirected line segment), but relative orientations around 45 degreesor 60 degrees can provide additional advantages. One or more other ofthe grating patterns arranged in one or more of the non-primaryorientations can include a third grating pattern having a third gratingvector. The second and third grating vectors can be inclined to thefirst vector by approximately the same amount in different directions.The second and third grating vectors respectively define a second pitchand a third pitch that can be equal to each other. The first gratingvector can define a first pitch that can be the same or different fromthe second and third pitches.

The out-coupling diffractive optic can be formed as a diffractionlattice containing a plurality of unit cells in a plane defining aplurality of diffraction patterns in different orientations within theplane. The unit cells can be in the shape of polygons such as rectanglesor hexagons for providing orderly relationships among the diffractionpatterns, including relative orientation and pitch. The diffractionpatterns provide for expanding each of the image-bearing beams in twodimensions for increasing areas of overlap within the eyebox.Preferably, the diffraction lattice comprises a plurality of diffractivefeatures that are arranged in a pattern and distinguished by arefractive index difference within the plane. The plane includeshorizontal and vertical dimensions and the diffractive features can havecorresponding horizontal and vertical dimensions that are not equal toregulate distributions of light within each of the image-bearing beams.Portions of the diffractive features can overlap, or the diffractivefeatures can be spaced apart from surrounding features. The diffractivefeatures can also be formed in the shape of polygons such as rectanglesor hexagons or can be formed as more rounded shapes such as circles,ellipses, or other oblong shapes.

For example, the plurality of diffraction patterns of the diffractionlattice can be arranged for diffracting portions of each image-bearinglight beam into at least three different directions. A first of thediffraction patterns diffracts first portions of the image-bearing lightbeams both from the waveguide toward the eye box in an angularly decodedform and along an original nominal direction of propagation in theiroriginal encoded form. The relative displacements of the first portionsof the image-bearing light beams in the original nominal direction ofpropagation contribute to expanding a first dimension of an area ofoverlap among the image-bearing light beams within the eyebox. A secondand a third of the diffraction patterns diffract second portions of theimage-bearing light beams in directions that are different from theoriginal nominal direction of propagation along the waveguide. Forexample, the second portions can be redirected near 45 degrees or near60 degrees from the original nominal direction of propagation. At leastone of the second and third diffraction patterns diffracts the secondportions back towards the original nominal propagation direction inlaterally offset positions along the waveguide. The relative lateraldisplacements of the second portions of the image-bearing light beamscontribute to expanding a second dimension of the area of overlap amongthe image-bearing light beams within the eyebox. Preferably,diffractions of the second portions that further angularly encode theimage-bearing beams, i.e., change the distributions of angularorientations among the image-bearing beams, are matched with furtherdiffractions of the second portions that decode such further encoding torestore the originally encoded distributions of angular orientationsamong the image-bearing beams.

The out-coupling diffractive optic with multiple grating patterns cantake a variety of forms. For example, the out-coupling diffractive opticcan be formed as volume holographic optical element (HOE) havingmultiple sub-elements. Such a volume HOE can be formed from avolume-recording medium with a plurality of holographic sub-elementsrecorded therein, overlapping within the volume of the medium.Considered separately, each sub-element of the volume HOE can functionas a diffraction grating having a grating vector in a directiondifferent from the other sub-elements comprising the total volume HOE.Alternatively, multiple, individual, holograms can be stacked one overthe other with each hologram recording a different diffraction gratingvector and, where required, a different grating period.

Similarly, multiple diffraction gratings can be stacked one over theother, each layer having a different grating vector orientation withrespect to one another. Where necessary, one of the stacked diffractiongratings can have a grating period different from the periods of theother diffraction gratings in the stack.

An image light guide for conveying a virtual image in accordance withone or more other embodiments includes a waveguide having a transmissivesubstrate with front and back surfaces for propagating image-bearinglight beams along a length of the waveguide. An in-coupling diffractiveoptic formed along the waveguide diffracts the image-bearing light beamsfrom an image source into the waveguide for propagating theimage-bearing light beams along the length of the waveguide in anangularly encoded form. An out-coupling diffractive optic spaced apartfrom the in-coupling diffractive optic along the length of the waveguidediffracts the image-bearing light beams from the waveguide in anangularly decoded form toward an eyebox for viewing the image from theimage source. The out-coupling diffractive optic is formed as adiffraction lattice having a plurality of grating features in a planedefining a plurality of diffraction patterns having differentorientations within the plane. Each of the grating features includes alength and width in the plane defining an aspect ratio of length towidth. At least some of the grating features have an aspect ratio thatdeparts from unity for altering diffraction efficiencies between thediffraction patterns.

The image-bearing beams approach the out-coupling diffractive opticalong a nominal direction of propagation. The length of the gratingfeatures can extend along the nominal direction of propagation and thewidth of the grating features can extend perpendicular to the directionof propagation. In one or more embodiments, the grating features havingan aspect ratio that departs from unity can have a length that exceedstheir width for enhancing the efficiency of diffractions that redirectportions of the image bearing beams in directions that depart from thenominal direction of propagation. The aspect ratio of the gratingfeatures having an aspect ratio that departs from unity can vary alongthe nominal direction of propagation. In addition, the grating featureshaving an aspect ratio that departs from unity can include gratingfeatures that extend contiguously along the nominal direction ofpropagation.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a simplified cross-sectional view of an image light guideshowing the expansion of an image-bearing beam along the direction ofpropagation for expanding one dimension of an eyebox.

FIG. 2 is a perspective view of an image light guide with a turninggrating showing the expansion of an image-bearing beam perpendicular tothe direction of propagation for expanding a second dimension of aneyebox.

FIG. 3 is a plan view of an image light guide showing a waveguide havingan in-coupling diffractive optic and out-coupling diffractive optic bothformed on the same surface of the waveguide.

FIG. 4 is a plan view showing an enlarged portion of the out-couplingdiffractive optic with a pattern of alternating grating vectors.

FIGS. 5A, 5B, 5C, 5D, and 5E show light behavior within individualgrating patterns of the out-coupling diffractive optic of FIG. 4.

FIG. 6 is a table that summarizes light handling for the patternedout-coupling diffractive optic of FIG. 4.

FIGS. 7A, 7B, and 7C show different geometrical arrangements of thesubsets of diffractive elements that are combined to form a diffractivearray according to embodiments of the present disclosure.

FIG. 8A is a plan view an image light guide with a waveguide havingin-coupling and out-coupling diffractive optics according to analternate embodiment, wherein a portion of the out-coupling diffractiveoptic is shown in an enlarged form.

FIG. 8B is a schematic diagram showing diffraction vectors fordiffractive elements of the waveguide of FIG. 8A.

FIG. 8C is a table that summarizes light handling for the patternedout-coupling diffractive optic of FIG. 8A.

FIG. 9A is a plan view of an image light guide having an arrangement ofdistribution gratings along a waveguide according to an embodiment ofthe present disclosure.

FIG. 9B is a similar plan view showing the gratings vectors of thedistribution gratings of FIG. 9A, including grating vectors within anenlarged portion of the out-coupling diffractive optic.

FIG. 10 is a perspective view showing light paths within the image lightguide of FIGS. 9A and 9B.

FIG. 11 is a flow diagram of light processing and distribution within animage light guide according to an embodiment of the present disclosure.

FIG. 12 is a perspective view showing a display system for augmentedreality viewing using image light guides of the present disclosure.

FIG. 13A, FIG. 13B, and FIG. 13C depict two diffraction gratingsindividually and in an overlapping form.

FIG. 14 is a schematic plan view of a compound diffraction gratingaccording to an embodiment of the present disclosure.

FIG. 15 is a schematic of three gratings patterns having differentorientations with respect to one another.

FIG. 16 is a schematic of the three grating patterns of FIG. 15 overlaidto form a compound grating pattern.

FIG. 17 is a schematic of the region of overlap from FIG. 16 selectedfor the compound grating pattern.

FIG. 18 is a schematic of another compound grating pattern having anarrangement of grating features in which the compound grating pattern isfurther defined by unit cells.

FIG. 19 is a schematic of yet another compound grating pattern definableby the replication of a unit cell.

FIGS. 20A, 20B, and 20C depict alternative forms of the unit cell ofFIG. 19 with FIG. 20D depicting various diffractive orders associatedwith assemblies of the variously formed unit cells.

FIGS. 21A, 21B, 21C, and 21D contain schematic depictions of successiveTIR encounters of an image-bearing beam along a waveguide from andin-coupling diffractive optic to an out-coupling diffractive opticshowing the effects of diffractions through a progression of diffractiveorders for expanding an image-bearing beam in two dimensions.

FIG. 22A and FIG. 22B depict respective perspective a plan views of animage light guide for expanding image-bearing beams in the form shown inFIGS. 21A through 21D.

FIG. 23 depicts grating vectors and diffractive orders associated with arectangular configuration of grating features.

FIG. 24A depicts a unit cell supporting an arrangement of diffractiveorders, and FIG. 24B is an assembly of unit cells according to FIG. 24Ainto a two-dimensional lattice showing propagations of the variousdiffractive orders throughout the represented compound grating pattern,whereby image-bearing beams are both expanded in two dimensions anddiffracted from the waveguide in a decoded form.

FIG. 25 is a schematic depiction of a compound grating pattern in whichthe grating features have an aspect ratio that departs from unity.

FIG. 26 is a schematic depiction of a compound grating pattern in whichthe grating features have an aspect ratio that progressively variesalong one dimension of the grating.

FIGS. 27A and 27B are SEM images showing examples of an as-manufacturedcompound grating patterns.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

Where they are used herein, the terms “first”, “second”, and so on, donot necessarily denote any ordinal, sequential, or priority relation,but are simply used to more clearly distinguish one element or set ofelements from another, unless specified otherwise.

In the context of the present disclosure, the terms “viewer”,“operator”, “observer”, and “user” are considered equivalents and referto the person who wears and views images using the HMD viewing device.

As used herein, the term “energizable” relates to the capability of adevice or set of components upon receiving power and, optionally, uponreceiving an enabling signal to perform an indicated function.

The term “set”, as used herein, refers to a non-empty set, as theconcept of a collection of elements or members of a set is widelyunderstood in elementary mathematics. The term “subset”, unlessotherwise explicitly stated, is used herein to refer to a non-emptyproper subset, that is, to a subset of the larger set, having one ormore members. For a set S, a subset may comprise the complete set S. A“proper subset” of set S, however, is strictly contained in set S andexcludes at least one member of set S.

In the context of the present disclosure, the term “oblique” means at anangle that is not an integer multiple of 90 degrees. Two lines, linearstructures, or planes, for example, are considered oblique with respectto each other if they diverge from or converge toward each other at anangle that is at least about 5 degrees or more away from parallel, or atleast about 5 degrees or more away from orthogonal.

The terms “coupled” or “coupler” in the context of optics refer to aconnection by which light travels from one optical medium or device toanother optical medium or device through an intermediate structure thatfacilitates the connection.

As an alternative to real image projection, an optical system canproduce a virtual image display. In contrast to methods for forming areal image, a virtual image is not formed on a display surface. That is,if a display surface were positioned at the perceived location of avirtual image, no image would be formed on that surface. Virtual imagedisplay has several inherent advantages for augmented reality display.For example, the apparent size of a virtual image is not limited by thesize or location of a display surface. In comparison with systems thatproject a real image, a more realistic viewing experience can beprovided by forming a virtual image that appears to be some distanceaway. Providing a virtual image also obviates any need to compensate forscreen artifacts, as may be necessary when projecting a real image.

FIG. 1 is a schematic diagram showing a simplified cross-sectional viewof one conventional configuration of a monocular type image light guide10 comprising a planar waveguide 22 having plane-parallel surfaces, anin-coupling diffractive optic IDO, and an out-coupling diffractive opticODO arranged on a transparent substrate S of the planar waveguide 22. Inthis example, in-coupling diffractive optic IDO is shown as a reflectivetype diffraction grating arranged on an inner surface 14 of the planarwaveguide 22 opposite to the outer surface 12 of the planar waveguide 22through which image-bearing light WI approaches the planar waveguide 22.However, in-coupling diffractive optic IDO could alternately be atransmissive diffraction grating, volume hologram or other holographicdiffraction element, or other type of optical component that providesdiffraction for the incoming, image-bearing light WI. The in-couplingdiffractive optic IDO can be located on the outer or inner surface 12 or14 of the planar waveguide 22 and can be of a transmissive or reflectivetype in a combination that depends upon the direction from which theimage-bearing light WI approaches the planar waveguide 22.

When used as a part of a virtual display system, in-coupling diffractiveoptic IDO couples the image-bearing light WI from a real, virtual orhybrid image source (not shown) into the substrate S of the planarwaveguide 22. Any real image or image dimension is first converted, e.g.converged toward a focus, into an array of overlapping angularly relatedbeams encoding the different positions within a virtual image forpresentation to the in-coupling diffractive optic IDO. The image-bearinglight WI is diffracted (generally through a first diffraction order) andthereby redirected by in-coupling diffractive optic IDO into the planarwaveguide 22 as image-bearing light WG for further propagation along theplanar waveguide 22 by Total Internal Reflection (TIR). Althoughdiffracted into a generally more condensed range of angularly relatedbeams in keeping with the boundaries set by TIR, the image-bearing lightWG preserves the image information in an encoded form. An out-couplingdiffractive optic ODO receives the encoded image-bearing light WG anddiffracts (also generally through a first diffraction order) theimage-bearing light WG out of the planar waveguide 22 as theimage-bearing light WO toward the intended location of a viewer's eye.Generally, the out-coupling diffractive optic ODO is designedsymmetrically with respect to the in-coupling diffractive optic IDO torestore the original angular relationships of the image-bearing light WIamong outputted angularly related beams of the image-bearing light WO.However, to increase one dimension of overlap among the angularlyrelated beams in a so-called eyebox E within which the virtual image canbe seen, the out-coupling diffractive optic ODO is arranged to encounterthe image-bearing light WG multiple times and to diffract only a portionof the image-bearing light WG on each encounter. The multiple encountersalong the length of the out-coupling diffractive optic ODO have theeffect of enlarging one dimension of each of the angularly related beamsof the image-bearing light WO thereby expanding one dimension of theeyebox E within which the beams overlap. The expanded eyebox E decreasessensitivity to the position of a viewer's eye for viewing the virtualimage.

The out-coupling diffractive optic ODO is shown as a transmissive typediffraction grating arranged on the inner surface 14 of the planarwaveguide 22. However, like the in-coupling diffractive optic IDO, theout-coupling diffractive optic ODO can be located on the outer or innersurface 12 or 14 of the planar waveguide 22 and be of a transmissive orreflective type in a combination that depends upon the direction throughwhich the image-bearing light WG is intended to exit the planarwaveguide 22.

The perspective view of FIG. 2 shows an image light guide 20 that isarranged for expanding the eyebox 74 in two dimensions, i.e., along bothx- and y-axes of the intended image. To achieve a second dimension ofbeam expansion, the in-coupling diffractive optic IDO is oriented todiffract the image-bearing light WG about a grating vector k0 toward anintermediate turning grating TG, whose grating vector k1 is oriented todiffract the image-bearing light WG in a reflective mode toward theout-coupling diffractive optic ODO. Only a portion of the image-bearinglight WG is diffracted by each of multiple encounters with intermediateturning grating TG thereby laterally expanding each of the angularlyrelated beams of the image-bearing light WG approaching the out-couplingdiffractive optic ODO. The turning grating TG redirects theimage-bearing light WG into an at least approximate alignment with agrating vector k2 of the out-coupling diffractive optic ODO forlongitudinally expanding the angularly related beams of theimage-bearing light WG in a second dimension before exiting the planarwaveguide 22 as the image-bearing light WO. Grating vectors, such as thedepicted grating vectors k0, k1, and k2, extend in a direction that isnormal to the diffractive features (e.g., grooves, lines, or rulings) ofthe diffractive optics and have a magnitude inverse to the period orpitch d (i.e., the on-center distance between grooves) of thediffractive optics IDO, TG, and ODO.

In the image light guide 20 of FIG. 2, in-coupling diffractive optic IDOreceives the incoming image-bearing light WI containing a set ofangularly related beams corresponding to individual pixels or equivalentlocations within an image generated by an image source 16. A full rangeof angularly encoded beams for producing a virtual image can begenerated by a real display together with focusing optics, by a beamscanner for more directly setting the angles of the beams, or by acombination such as a one-dimensional real display used with a scanner.The image light guide 20 outputs an expanded set of angularly relatedbeams in two dimensions of the image by providing multiple encounters ofthe image-bearing light WG with both the intermediate turning grating TGand the out-coupling diffractive optic ODO in different orientations. Inthe original orientation of the planar waveguide 22, the intermediategrating TG provides beam expansion in the y-axis direction, and theout-coupling diffractive optic ODO provides a similar beam expansion inthe x-axis direction. The reflectivity characteristics and respectiveperiods d of the two diffractive optics IDO and ODO and the intermediategrating TG, together with the orientations of their respective gratingvectors, provide for beam expansion in two dimensions while preservingthe intended relationships among the angularly related beams of theimage-bearing light WI that are output from the image light guide 20 asthe image-bearing light WO.

That is, while the image-bearing light WI input into the image lightguide 20 is encoded into a different set of angularly related beams bythe in-coupling diffractive optic IDO, the information required toreconstruct the image is preserved by accounting for the systematiceffects of the in-coupling diffractive optic IDO. The turning gratingTG, located in an intermediate position between the in-coupling andout-coupling diffractive optics IDO and ODO, is typically arranged sothat it does not induce any significant change on the encoding of theimage-bearing light WG. The out-coupling diffractive optic ODO istypically arranged in a symmetric fashion with respect to thein-coupling diffractive optic IDO, e.g., including diffractive featuressharing the same period. Similarly, the period of the turning grating TGalso typically matches the common period of the in-coupling andout-coupling diffractive optics IDO and ODO. Although the grating vectork1 of the turning grating is shown oriented at 45 degrees with respectto the other grating vectors (all as undirected line segments), whichremains a possible orientation, the grating vector k1 of the turninggrating TG is preferably oriented at 60 degrees to the grating vectorsk0 and k2 of the in-coupling and out-coupling diffractive optics IDO andODO in such a way that the image-bearing light WG is turned 120 degrees.By orienting the grating vector k1 of the intermediate turning gratingat 60 degrees with respect to the grating vectors k0 and k2 of both thein-coupling and out-coupling diffractive optics IDO and ODO, the gratingvectors k0 and k2 of the in-coupling and out-coupling diffractive opticsIDO and ODO are also oriented at 60 degrees with respect to each other(again considers as undirected line segments). Basing the grating vectormagnitudes on the common pitch of the turning grating TG and thein-coupling and out-coupling diffractive optics IDO and ODO, the threegrating vectors k0, k1, and k2 (as directed line segments) form anequilateral triangle, and sum to a zero-vector magnitude, which avoidsasymmetric effects that could introduce unwanted aberrations includingchromatic dispersion.

The image-bearing light WI that is diffracted into the planar waveguide22 is effectively encoded by the in-coupling optic, whether thein-coupling optic uses gratings, holograms, prisms, mirrors, or someother mechanism. Any reflection, refraction, and/or diffraction of lightthat takes place at the input must be correspondingly decoded by theoutput to re-form the virtual image that is presented to the viewer.Preferably, the turning grating TG, placed at an intermediate positionbetween the in-coupling and out-coupling diffractive optics IDO and ODO,is typically designed and oriented so that it does not induce any changeon the encoded light. Out-coupling diffractive optic ODO decodes theimage-bearing light WG into its original or desired form of angularlyrelated beams that have been expanded to fill the eyebox 74. In abroader sense, whether any symmetries are maintained or not among theturning grating TG and the in-coupling and out-coupling diffractiveoptics IDO and ODO or whether or not any change to the encoding of theangularly related beams of the image-bearing light WI takes place alongthe planar waveguide 22, the turning grating TG and the in-coupling andout-coupling diffractive optics IDO and ODO are related so that theimage-bearing light WO that is output from the planar waveguide 22preserves or otherwise maintains the original or desired form of theimage-bearing light WI for producing the intended virtual image.

The letter “R” represents the orientation of the virtual image that isvisible to the viewer whose eye is in the eyebox 74. As shown, theorientation of the letter “R” in the represented virtual image matchesthe orientation of the letter “R” as encoded by the image-bearing lightWI. A change in the rotation about the z axis or angular orientation ofincoming image-bearing light WI with respect to the x-y plane causes acorresponding symmetric change in rotation or angular orientation ofoutgoing light from out-coupling diffractive optic (ODO). From theaspect of image orientation, the turning grating TG simply acts as atype of optical relay, providing expansion of the angularly encodedbeams of the image-bearing light WG along one axis (e.g., along the yaxis) of the image. Out-coupling diffractive optic ODO further expandsthe angularly encoded beams of the image-bearing light WG along anotheraxis (e.g., along the x axis) of the image while maintaining theoriginal orientation of the virtual image encoded by the image-bearinglight WI. The turning grating TG is typically a slanted or squaregrating or, alternately, can be a blazed grating and is typicallyarranged on the front or back surfaces of the planar waveguide 22.

The image light guides 10 and 20 depicted in FIGS. 1 and 2 can be of atype used in many existing head-mounted device (HMD) designs forproviding image content to a viewer. This type of image light guide isparticularly well suited to augmented reality applications in whichvirtual image content can be superimposed on a real-world view as seenthrough the transparent planar waveguide 22.

The conventional image light guide 20 that is shown in FIGS. 1 and 2 isused in many existing head-mounted device (HMD) designs for providingimage content to a viewer. This type of light guide is particularly wellsuited to augmented reality applications in which image content can besuperimposed on a real-world view as seen through the transparent imagelight guide.

An embodiment of the present disclosure provides improved waveguideperformance and eliminates the need for a separate turning grating TG inthe light path by using a diffractive array that expands each of theimage-bearing light beams in both x and y directions and outputs theexpanded image-bearing beams toward an eyebox.

Referring to FIG. 3, there is shown a waveguide as an image light guide150 having in-coupling diffractive optic IDO and out-couplingdiffractive optic ODO, both IDO and ODO that can be formed on the samesurface of waveguide 150. Alternately, IDO and ODO can be formed on thedifferent surfaces of waveguide 150. In-coupling diffractive optic IDOhas the grating vector k extending in the x direction, as shown.Out-coupling diffractive optic ODO has multiple grating vectors that arenot parallel to grating vector k, as shown subsequently.

The plan view of FIG. 4 shows an arrangement of out-coupling diffractiveoptic ODO as a diffractive array 100. Array 100 has multiple componentdiffractive optical elements or optics 102. In a row of the array 100,sequential diffractive optical elements 102 have alternating gratingvectors k2 and k3. The alternating grating vector arrangement isschematically shown for a portion of array 100 in an enlarged view OE.Grating vectors k2 are offset from input grating vector k and from axisx by +60 degrees (alternately, offset from axis y by −30 degrees).Grating vectors k3 are offset from input grating vector k and axis x by−60 degrees.

Diffractive array 100, used in the light path as out-couplingdiffractive optic in FIG. 4 and subsequent embodiments, can beconsidered structurally formed as the union of disjoint, mutuallynon-overlapping subsets of diffractive elements or optics formed on asingle surface. Considered in terms of set theory, this union of subsetsforms a “partition”. There is a unique grating vector corresponding toeach subset of the partition and the subsets are distinguished from eachother according to the grating vector direction. That is, all thediffractive elements in each subset have a common grating vector. In thespatial arrangement of diffractive elements 102, elements of at leasttwo subsets alternate with each other, so that each element from thesubset with grating vector k2 is immediately adjacent to one or moreneighboring elements from the other subset with grating vector k3. Morethan two subsets of immediately adjacent elements can be used toconstitute the partition of diffractive array 100; each subset has agrating vector that extends in a different direction from thecorresponding grating vector for any other subset.

The schematic views of FIGS. 5A through 5E show how this arrangementusing alternating grating vectors operates at individual diffractiveelements 102 to distribute and output light for forming a virtual image.FIG. 5A shows how light incident from IDO, traveling toward the left inthe depiction shown, interacts over a portion of diffractive element 102that is formed having grating vector k2. Some portion of the incidentlight is diffracted at optic 102 and thereby deflected 120 degrees fromthe original path of light from IDO. A percentage of this light isdiffracted and redirected, to other portions of array 100, at each ofseveral points along the diffraction grating. Another portion of thislight is not diffracted, but instead travels through the diffractiveoptical element 102 to the next optical element 102 in sequence.

FIG. 5B shows a similar path to that described for FIG. 5A, but in theinverse direction. Incident light from IDO that is conveyed todiffractive element 102 having grating vector k3 can take either of theangular paths shown. Some portion of the incident light is diffracted atoptical element 102 and deflected −120 degrees from the original path oflight from IDO. A percentage of this light is diffracted and redirected,to other portions of array 100, at each of several points along thediffraction grating. Another portion of this light is not diffracted,but instead travels through the diffractive element 102 and to the nextelement 102 in sequence.

FIG. 5C shows what happens when light is incident at an angle that issubstantially orthogonal to the gratings of diffractive element 102. Atthis angle, the light exits from diffractive element 102, with someportion of the light directed outward at each interaction with thediffractive structures.

FIG. 5D shows one type of interaction that causes light to be outputfrom array 100. Some of the light diffracted from element 102 havinggrating vector k2 can be incident on another optical element 102 furtherinto the array 100 and having grating vector k3. The condition shown inFIG. 5C is satisfied, with light incidence parallel to the gratingvector. This light is then output (that is, output from the page in theview of FIG. 5D).

FIG. 5E shows a different interaction that can occur along the lightpath within array 100. Here, some of the light diffracted from opticalelement 102 having grating vector k2 can be incident on another element102 further into the array 100 also having grating vector k2. Theresulting output light is directed outward, parallel to the input lightin the arrangement shown.

FIG. 6 shows Table 1 that summarizes the predominant light response foreach diffractive element 102 of a diffractive array 100. From thedescription related to FIGS. 5A-6 numerous potential paths are providedfor incident light within diffractive array 100, allowing the light tobe spread along both x and y axis directions for providing an expandedoutput. However, not all the light incident at a particular angle isredirected as shown in FIG. 6. Light that is not redirected at firstcontact with the grating can continue through the grating, to beredirected at any number of additional points along the grating. FIG. 6shows the predominant path for light that will form the viewer image.

There are many shape arrangements for diffractive elements 102,depending on factors such as needed performance and manufacturability.The embodiment of FIG. 7A shows an enlarged portion of a checkerboardconfiguration, using rectangular elements 102 arranged with gratingvectors k2, k3 alternating along each row and along each column of thearray 100. FIG. 7B shows an enlarged portion of an arrangement usinghexagons. FIG. 7C shows an enlarged portion of an arrangement usingtriangles. For each of these arrangements, the diffractive elements 102are of either a first subset having a grating vector that is offset fromthe grating vector of the IDO by about +60 degrees (that is, offset byan angle within +60+/−3 degrees), or a second subset having a gratingvector that is offset from the grating vector of the IDO by about −60degrees (that is, offset by an angle within −60+/−3 degrees). Eachdiffractive element of the first (+60 degree) subset is immediatelyadjacent to at least one diffractive element of the second (−60 degree)subset. The diffractive elements of the first and second subsets aremutually non-overlapping.

Design parameters such as gratings depth and other features can bevaried across the grating, such as to selectively output higherpercentages of light toward extreme portions of out-coupling diffractiveoptic, for example.

In addition to shape modification, other changes to the basic model ofFIG. 4 can include using, within diffractive array 100, some number ofadditional grating elements of a subset having grating vectors at anglesother than +/−60 degrees. According to an embodiment of the presentinvention, some amount of randomization can be provided, including useof gratings at 0 degrees or at some other suitable angle, selectivelyinterspersed in the light path extending along the array 100.

FIG. 8A and the enlarged schematic view of FIG. 8B show an image lightguide 170 having in-coupling and out-coupling diffractive optics on thesame side of the waveguide substrate according to an alternateembodiment of the present disclosure. In the configuration shownout-coupling diffractive optic ODO is formed of groupings of threediffractive elements, immediately adjacent to each other and havingthree corresponding grating vectors k, k2, and k3 as shown in FIG. 8B.The table in FIG. 8C shows light handling behavior for light travelingat different angles within the image light guide 170. Grating vector kis about 60 degrees (60 degrees+/−3 degrees) from either grating vectorsk2 and k3.

Advantageously, the embodiment shown in FIGS. 4 and 8A provides awaveguide that provides beam expansion without requiring turninggratings. Light distribution, however, can be improved using additionaldistribution gratings, as shown in the exemplary embodiments of FIGS.9A, 9B, and 10.

The plan view of FIG. 9A shows a waveguide that provides an image lightguide 160 using an arrangement of diffractive optics to form an expandedoutput beam from the waveguide. Light from in-coupling diffractive opticIDO is directed to first and second distribution gratings 70 forexpansion with respect to the y dimension shown. The resulting light isthen redirected into out-coupling diffractive optic ODO by paireddistribution gratings 72. Out-coupling diffractive optic ODO usesdiffractive array 100 as described previously. FIG. 9B showsrepresentative grating vectors for the different diffractive componentsof the image light guide 160.

FIG. 10 shows a schematic view of light distribution paths that aredefined for image-bearing light directed to in-coupling diffractiveoutput IDO by light guide 160 of FIGS. 8 and 9. Light from in-couplingdiffractive output IDO is expanded along the direction of the y-axis,with the light path directed through one or both distribution gratings70. Light from each distribution grating 70 is then conveyed to thesecond pair of distribution gratings 72. Distribution gratings 72redirect the light internally, but do this to spread the beam,redirecting the light in the direction of the output light path, towardout-coupling diffraction grating ODO. Out-coupling diffraction gratingODO then directs the light orthogonally out of the image light guide 160to form a viewer eyebox. Thus, the path of image-bearing light can bedirected through at least two distribution gratings, wherein thedistribution gratings are spaced apart from each other and expand theimage-bearing light within the image light guide.

Pass-through light that proceeds along the light path from grating 70 tothe subsequent distribution grating 72 is light that has been diffractedan even number of times. Expansion of the light with respect to the yaxis is a result of odd-numbered diffractions {1, 3, 5, etc. . . . ]}.

The flow chart shown in FIG. 11 describes how image light guide 150handles light with the arrangement of gratings shown in FIGS. 8-10. Inan input step 400, light is input to in-coupling diffractive optic IDO.The light proceeds to distribution grating 70 in step 410. In step 420,diffracted light from grating 70 then proceeds to a turn step 424,wherein the light is turned to either +60 degrees or −60 degrees. Lightturned −60 degrees proceeds to a diffraction step 430 in diffractiongrating 72. Undiffracted light is cycled in step 450. Diffracted lightproceeds to a turn step 470 and is redirected at either +/−60 degrees.Similarly, light turned +60 degrees proceeds to a diffraction step 440in diffraction grating 72. Undiffracted light is cycled in step 460.Diffracted light proceeds to a turn step 480 and is redirected at either+/−60 degrees. Light is output from out-coupling diffractive optic ODOin an output step 490. It should be noted that FIG. 11 identifies thepredominant light path, approximating the light handling behavior formuch of the light within the image light guide. Not all the lightundergoes diffraction at various interfaces, for example.

The perspective view of FIG. 12 shows a display system 60 forthree-dimensional (3-D) augmented reality viewing using a pair of imagelight guides of the present disclosure. Display system 60 is shown as anHMD with a left-eye optical system 541 having an image light guide 1401for the left eye and a corresponding right-eye optical system 54 rhaving an image light guide 140 r for the right eye. An image source 52,such as a picoprojector or similar device, can be provided, energizableto generate a separate image for each eye, formed as a virtual imagewith the needed image orientation for upright image display. The imagesthat are generated can be a stereoscopic pair of images for 3-D viewing.The virtual image that is formed by the optical system can appear to besuperimposed or overlaid onto the real-world scene content seen by theviewer. Additional components familiar to those skilled in the augmentedreality visualization arts, such as one or more cameras mounted on theframe of the HMD for viewing scene content or viewer gaze tracking, canalso be provided. Alternate arrangements are possible, including adisplay apparatus for providing an image to one eye.

FIG. 13A is a schematic of a diffraction grating 500 having a gratingvector k1 and a grating period d1 defining a first grating pattern. Inone embodiment, gating vector k1 has an angle of 45 degrees with respectto the x-direction. In another arrangement, grating vector k1 has anangle of approximately 60 degrees with respect to the x-direction. Inyet another arrangement, grating vector k1 has an angle of approximately30 degrees with respect to the x-direction.

FIG. 13B is a schematic of a diffraction grating 510 having a gratingvector k2 and a period d2 defining a second grating pattern. In onearrangement, gating vector k2 has an angle of −45 degrees with respectto the x-direction. In another arrangement, grating vector k2 has anangle of approximately −60 degrees with respect to the x direction. Inanother arrangement, grating vector k2 has an angle of approximately −30degrees with respect to the x-direction.

FIG. 13C is a schematic of an overlapped diffraction grating 520composed of overlapping diffraction grating 500 and diffraction grating510. In one arrangement, the angular relation between grating vectors k1and k2 is 45 degrees. In another arrangement, the angular relationbetween grating vectors k1 and k2 is approximately 60 degrees. Inanother arrangement, the angular relation between grating vectors k1 andk2 is approximately 30 degrees. In another arrangement, the angularrelation between grating vector k1 and grating vector k2 is between 0and 180 degrees.

In one arrangement, the period d1 of the first diffraction grating isthe same as the period d2 of the second diffraction grating. In anotherarrangement, the period d1 of the first diffraction grating is differentfrom the period d2 of the second diffraction grating. In one arrangement(not shown), at least one of the diffraction patterns has a chirpedperiod that changes in the direction of the grating vector.

Conical diffraction rules encompass arbitrary angles of incidence,including angle of incidence having an azimuthal component out of theplane that is normal to the grating and includes the grating vector. Amathematical description of these rules formulated for direction cosinespace is provided in a paper entitled “Description of DiffractionGrating Behavior in Direction Cosine Space” by James E. Harvey, andCynthia L. Vernold, Applied Optics, Vol. 37, Iss. 34, pp. 8158-8160(1998), which paper is hereby incorporated by reference.

For purposes of defining conical diffractions, a right-handed coordinatesystem can be centered on the intersection point of the input ray vectorwith the plane of the grating, such that the grating's normal vector isalong the z-axis, the x-axis is along the input ray's vector projectedonto the xy-plane. Then the x- and y-coordinates of the m^(th) orderdiffracted ray projected onto the xy-plane are given by the equations:

${x_{m} = {{\left( \frac{m\; \lambda}{d} \right){\cos (\kappa)}} - \left\lbrack {{\sin \left( \theta_{i\; n} \right)}{\cos \left( \varphi_{i\; n} \right)}} \right\rbrack}},\; {y_{m} = {{\left( \frac{{m\; \lambda}\;}{d} \right){\sin (\kappa)}} - \left\lbrack {{\sin \left( \theta_{i\; n} \right)}{\sin \left( \varphi_{i\; n} \right)}} \right\rbrack}},$

where m=0, ±1, ±2, . . . indicates the diffraction order, λ=λ₀/n_(in) isthe wavelength in the material that the light ray is traveling through,the material having an index of refraction n_(in) and with λ₀ being thewavelength of the light ray in vacuum, d is the grating's pitch, κ(Greek letter kappa) being the angle of the grating vector k withrespect to the positive x-axis direction, θ_(in) is the polar angle thatthe incident ray makes with respect to the z-axis (the grating'snormal), and ϕ_(in) is the azimuth angle that the projection of theincident ray makes with respect to the x-axis in the xy-plane. For thecase that the incident ray is in the xz-plane, coming from the −x-axistoward the +x-axis direction, the azimuth angle will be ϕ_(in)=0°.

The condition that

r ² ≡x _(m) ² +y _(m) ²≤1,

ensures that the diffracted ray is real (as opposed to an evanescentray).

Then, the azimuth angle in the xy-plane, with respect to the x-axis, forthe m^(th) order diffracted ray is determined by:

$\varphi_{out} = {{arc}\; {{\cos \left( \frac{x_{m}}{r} \right)}.}}$

In this way, the turning of the diffracted ray can be determined. Thepolar angle that the m^(th) order diffracted ray makes with the z-axisis determined by:

θ_(out)=arcsin(r).

These equations may be used to determine a first order design of linearcrossed gratings by considering the multiple crossed grating vectorsindividually as non-crossed gratings. Because other, secondary, gratingvectors may occur when designing crossed gratings, additional adjustmentof the design, based in part on experimentation, and/or a more rigorousFourier analysis of the completed cross grating, may be desired for aparticular application of the crossed gratings being considered.

FIG. 14 is a schematic of a compound diffraction grating 600 that iscomposed of three overlapping gratings. As a compound grating, the threegratings overlap in the same plane. The separate diffraction gratingsare demarcated by three grating vectors k1, k2, and k3, defining threedifferent grating patterns. The period d of each of the three gratingsis the same value. In other arrangements, the three grating periods arenot all the same. In addition, at least one of the three grating periodscan have a chirped period. As shown, all three grating vectors k1, k2,and k2 are related by angles of 60 degrees (when considered asundirected line segments). Basing the grating vector magnitudes on acommon pitch, the three grating vectors k1, k2, and k3 (as directed linesegments) form an equilateral triangle and sum to a zero magnitude. Inother arrangements, the grating vectors can be relatively oriented bydifferent angular amounts. For example, in an arrangement in which theimage-bearing beams approach the compound diffraction grating in anominal direction of propagation in alignment with the grating vectork1, the grating vectors k2 and k3 could be oriented with respect to thegrating vector k1 by angles of 45 degrees.

FIG. 15 is a schematic of three diffraction grating patterns 610, 612,614 each oriented at a different angle from one another like the gratingpatterns of FIG. 14. In a binary arrangement, the darkened bars 620represent the removal of material and the white spaces 622 between thedarkened bars 620 represent original material from which a compounddiffraction grating can be made.

FIG. 16 is a schematic showing the three diffraction grating patterns610, 612, 614 of FIG. 15 overlapping one another. From the overlappingpatterns, a region 650 is selected as the final diffraction pattern of acompound diffraction grating. For purposes of illustration, the periodsof the gratings are greatly enlarged, and the numbers of periods aregreatly reduced.

FIG. 17 is an enlarged schematic of the region 650 showing the regions652 where material is removed and regions 654 where the originalsubstrate material remains. The remaining regions 654 can be referred toas grating features, and with respect to this arrangement of gratingpatterns, the grating features have a hexagonal shape. In otherdistributions of removed material to remaining material, the gratingfeatures can take other shapes.

FIG. 18 shows a schematic of a compound grating pattern 660 formed withthree overlapping diffraction patterns. The dark regions 664 representareas where original material has been removed to form the grating 660.The remaining regions contain hexagonal grating features 654. Here, thegrating patterns are formed by the arrangements of the grating featuresthemselves based on the replication of unit cells into a two-dimensionallattice. For example, the depicted patterns can be formed by thereplication of a rectangular unit cell 666 or a hexagonal unit cell 668.Either of the unit cells 666 or 668 can define the entire compoundgrating pattern 660 by contiguous replication of the respective unitcells with adjacent unit cells sharing vertices within thetwo-dimensional lattice. Although the grating vectors k1, k2, and k3 arerelatively oriented with respect to each other through 60 degrees likethe compound grating pattern 650 of FIG. 17, the unit cell definitionsof the compound grating pattern 660 provide additional freedoms forfurther shaping and orienting the grating features 662 within the sameoverall grating patterns. For example, the same relative orientationsand periods of the grating vectors k1, k2, and k3 would remain unchangedeven if the grating features 662 were shaped as squares, rectangles,circles or ovals. In addition, the grating features 662 can be definedby either material remaining following a machining operation or materialremoved by a machining operation. More broadly, the grating features canbe defined by an optical property that differentiates the gratingfeatures from their surroundings, such as a difference in refractiveindex.

FIG. 19 depicts another compound grating pattern 700 including twooverlapping grating patterns demarcated by generally orthogonal gratingvectors k1 and k2 based on a rectangular unit cell 702. In thearrangement shown, grating vector k1 extends generally parallel to the xcoordinate axis, and the grating vector k2 extends generally parallel tothe y coordinate axis. A grating feature 704 is defined within eachrectangular unit cell.

While the compound grating patter 700 can still be defined as atwo-dimensional lattice, the grating feature 704 spans one dimension ofthe unit cell 702 along the x coordinate axis such that the gratingfeature 704 within the rectangular unit cells 702 is contiguous with thegrating features 704 of adjoining unit cells 702 along the x coordinateaxis. As such, the periodicity of the compound grating pattern in the k1vector direction is created by regular variations of the grating feature704 in the y coordinate direction. The regular variation defining theperiodicity of the k1 grating vector is a saw tooth pattern. Theperiodicity of the compound grating pattern in the k2 vector directionis created by the regular or average spacing between the rows ofcontiguous grating features in the y coordinate direction.

Notably, the grating pattern defined by periodic variations in the ycoordinate direction associated with the k2 grating vector are morepronounced than the grating pattern defined by periodic variations inthe x coordinate direction associated with the k1 grating vector.Considered as grating lines, the individual grating lines associatedwith the k1 grating vector extend discontinuously along the y coordinateaxis whereas the individual grating lines associated with the k2 gratingvector extend more continuously along the x coordinate axis. Thisstructural disparity favors diffractions associated with the k2 vectorover diffractions associated with the k1 grating vector for enhancingthe spread (i.e., expansion) of the image bearing beams along the ycoordinate axis.

FIGS. 20A, 20B, and 20C depict three alternative rectangular unit cells710, 714, and 718 with differently shaped grating features 712, 716, 720that extend contiguously along the x coordinate axis. While the gratingfeature 716 of the rectangular unit cell 714 is like the grating feature704 of the rectangular unit cell 702 of FIG. 19, the grating feature 712of the rectangular unit cell 710 is closer to a unit of a square wave,and the grating feature 720 of the rectangular unit cell 718 is closerto sinusoidal wave.

However, as shown diagrammatically in FIG. 20D, all three rectangularunit cells 710, 714, and 718 define compound grating patterns withorthogonal grating vectors k1 and k2. The arrow 722 represents a nominaldirection of propagation among the image-bearing beams approaching theschematic rectangular unit cell 724, which defines the unit vectors k1and k2, resulting in the diffraction of light through various angles.Light diffracted through the zero order along arrow 726 continues topropagate by TIR along the waveguide without change until encounteringthe same or similar unit cell pattern structure elsewhere within thecompound grating pattern along the direction of propagation. In the x-yplane of the waveguide, the portions of the image-bearing beamsdiffracted through the zero order represented by the arrow 726 maintaintheir original directions of propagation.

Various first order diffractions are possible based on the compounddiffraction grating pattern. For example, portions of the image-bearingbeams are diffracted out of the waveguide as depicted by the circle 728primarily based on an encounter with the grating pattern defined by thek1 vector like a conventional out-coupling diffractive optic. Thegrating vector k1 preferably matches the grating vector of thein-coupling diffractive optic so that the image-bearing beams ejectedfrom the waveguide are decoded into their original angularly relatedform. However, the compound grating pattern also produces other firstorder diffractions depicted by arrows 730 and 732 along diagonals of theunit cell 724, which spread portions of the image-bearing beams out ofthe nominal direction of propagation. The so diffracted portions remainwithin the waveguide but can be angularly encoded with respect to eachother in a different form, i.e., in a form different from the angularlyencoded form imposed by the in-coupling diffractive optic. However, theoriginal angular encoding can be restored by a subsequent encounter withthe same or similar unit cell structure elsewhere in the compoundgrating pattern.

Unlike in hexagonal grid patterns, the y vector of these gratings isindependent of the x vector. Image-bearing beam portions redirected by adiffraction order into the waveguide along vectors not parallel to thenominal ray direction of travel can be encoded into a new set of angles.However, a second interaction with an equivalently pitched pattern (in xand y dimensions) effectively unencodes the outcoupled beam portions orreturns the beam portions to their prior encoding for travel along theiroriginal direction within the waveguide.

FIGS. 21A through 21D depict patterns of successive encounters with anout-coupling diffractive optic 750 defined by a grating structure basedon the unit cell of FIG. 20. The out-coupling diffractive optic 750 withoverlapping grating patterns performs two primary functions. In additionto ejecting the image-bearing beams from the waveguide, the out-couplingdiffractive optic 750 expand the image-bearing beams in two dimensionsto increase their common area of overlap within the eyebox. Theimage-bearing beams are expanded by successions diffractive encounterswith the out-coupling diffractive optic, each encounter diffracting theimage-bearing beams through different diffractive orders.

FIG. 21A depict successions of encounters of an image-bearing beampropagating by TIR along the waveguide from an in-coupling diffractiveoptic 740 to the out-coupling diffractive optic 750. In FIG. 21A, onlyzero-order diffractions from the out-coupling diffractive optic areshown. Each such encounter, which is equivalent to a TIR reflection, isshown represented by a circle with a centered cross. In the furthercourse of propagation, the zero-order diffracted light reflects from theopposite side surface of the waveguide for subsequent encounters withthe out-coupling diffractive optic in positions displaced along theoriginal direction of propagation. Thus, the zero-order diffractionsfurther propagate the considered image-bearing beam along the originaldirection of propagation as projected onto the x-y plane of thewaveguide.

FIG. 21B shows the effect of the first order diffraction designated bycircle 716 in FIG. 20D, where decoded light is diffracted upon eachencounter out of the waveguide toward the eyebox. The ejected light isdepicted as a darkened circle overlaid with an “X”. Thus, theimage-bearing beam is expanded in the nominal direction of propagationas a succession of relatively displace beamlets, each composed of aportion of the light within one of the original image-bearing beam. Thek1 grating vector pattern responsible for such diffractions preferablymatches the grating vector orientation and magnitude of the in-couplingdiffractive optic to decode the angular relationships imposed upon theimage-bearing beams by the in-coupling diffractive optic in a mannerequivalent to reversing the direction through which the image-bearingbeams originally diffracted from the in-coupling diffractive optic.

FIG. 21C shows the addition of the first order diffractions designatedby the arrows 718 and 720 of FIG. 20D. The additional first orderdiffractions 718 and 720 diffract portions of the image-bearing beam indifferent directions that are inclined to the nominal direction ofpropagation as projected onto the x-y plane of the waveguide. Eachencounter being subject to the additional first order diffractionsspreads portions of the image-bearing beam in transverse directionsthroughout the waveguide. Zero-order diffractions also contribute to thespread of the successive encounters by maintaining the instant directionof the additional first order diffractions 718 and 720 throughsuccessive encounters.

Each initial diffraction through one of the additional diffractionorders directs a portion of the image-bearing beam out of the nominaldirection of propagation. In doing so, the affected portions of theimage-bearing beams can be further encoded into a different angularlyrelated form. However, a subsequent diffraction through the samediffractive order essentially operating in reverse can restore both theoriginal angular encoding and the original direction of propagationamong the diffracted portions of the image-bearing beams. Although boththe encoding and direction of the beam portions are restored, the beamportions are offset in a direction normal to the nominal direction ofpropagation, having the effect of expanding the image-bearing beams in asecond dimension.

FIG. 21D shows the effect of the first order diffraction designated bycircle 716 in FIG. 20D within the expanded range of encounters along thesecond dimension. Encounters of the beam portions restored inorientation and encoding include the first order diffractions 716 thateject decoded light from the waveguide into the eyebox so that theimage-bearing beams overlap within a common area of the eyebox that isexpanded in two dimensions.

FIGS. 22A and 22B show different views of a waveguide 760 with thegrating patterns of an in-coupling diffractive optic 762 and anout-coupling diffractive optic 764 depicted schematically. The gratingvector k0 of the in-coupling diffractive optic 762 is aligned with oneof the grating vectors of the out-coupling diffractive optic 764, andthe period d0 of the in-coupling diffractive optic 762 matches theperiod d1 of the out-coupling diffractive optic 764. Thus, the encodingof the image-bearing beams diffracted by the in-coupling diffractiveoptic 762 is decoded by the out-coupling diffractive optic 764 so thatthe angular relationships among the image-bearing beams (e.g. 766)presented to the in-coupling diffractive optic 762 is restored among theimage-bearing beams (e.g., 768) ejected from the out-couplingdiffractive optic 764. However, the overlapping grating patterns of theout-coupling diffractive optic 764 also provide for expanding the areaof overlap among the ejected beams (e.g., 768) over two dimensions.

FIG. 23 shows the layout of a compound grating pattern 780 in whichsquare grating features 782 are laid out in a rectangular grid havingorthogonal grating vectors k1 and k2. The various orders of diffractionare depicted for distributing light both along the nominal direction ofpropagation as well as in opposite directions that include componentsperpendicular to the nominal direction.

FIG. 24A depicts the diffractive orders of a unit cell 800, which asassembled into the array of FIG. 24B defines a compound grating pattern810 that can be used as an out-coupling diffractive optic for ejectingdecoded image-bearing beams from a waveguide in a form that expands andarea of overlap between the image-bearing beams in two dimensions. As anarray based on the common unit cell 800, the various diffractive ordersthat remain within the waveguide propagate in directions aligned withcomplementary diffractive orders of the adjoining unit cells 800. Thus,at each successive encounter, the remaining portion of the image-bearingbeam is diffracted through the same set or a subset of directions. Anyfurther encoding imparted by diffraction out of the nominal direction ofpropagation can be remove by an equivalent diffraction that restoresboth the desired encoding and the further nominal direction ofpropagation at which one of the diffractive orders can be arranged forejecting the beam portions out of the waveguide for filling an eyebox.In the same way, the diffraction pattern that directs light out of thewaveguide into the eyebox is preferably equivalent to the diffractionpattern of the in-coupling diffractive optic so that the encodingimposed upon the image-bearing beams upon diffraction into the waveguideis decoded by the diffraction of the same beams out of the waveguide.

FIG. 25 depicts a compound grating pattern 820 in the form of atwo-dimensional lattice within which four grating patterns have beenidentified as represented by grating vectors k1, k2, k3, and k4 forvariously diffracting a set of image-bearing beams propagating from anin-coupling diffractive optic in the nominal direction represented bythe arrow 822. As shown, each of the grating features 824 has an aspectratio of length l to width w of greater than unity. Increasing theaspect ratio of the grating features 824, such as into ellipsoidal orrectangular shapes, along the nominal direction 822 of input beamsreduces the energy into the out-coupled diffraction orders whileincreasing the energy into the turned diffraction orders for favoringexpansion of the input beams along the y coordinate axis. As the inputbeams encounter the compound grating 820 along the nominal direction ofpropagation 822, too much energy can be directed through theout-coupling diffractive orders, producing uneven illumination withinthe eyebox. Relatively reducing the width w of the grating features 824with respect to their length l contributes to filling the eyebox withmore even illumination.

FIG. 26 depicts another compound grating 830 in the form of atwo-dimensional lattice defined by grating vectors oriented like thegrating vectors k1, k2, k3, and k4 of the compound grating pattern 820of FIG. 25. The input beams are also presented to the compound grating830 in a similar nominal direction of propagation 832. However, theaspect ratio of the grating features 834 progressively varies in thenominal direction of propagation 832. As shown, the width w of thegrating features 834 remain the same. However, the lengths of thegrating features 834, which start considerably longer than the width ofthe grating features 834, progressively reduce in length along thenominal direction of propagation 832. Such progressively varying aspectratios of the grating features 834 can also be used to further balancethe out-coupled beam brightness levels throughout the eyebox. Once theimage-bearing beams have expanded across the output aperture area of thewaveguide along the y coordinate axis, a more balanced (closer to 1:1)aspect ratio can be used to efficiently out-couple the remainingportions of the image-bearing beams within the output aperture.

Thus, in addition to the objective of expanding the image-bearing beamsoutput from the out-coupling diffractive optic along both the x and ycoordinate axes to increase their area of overlap within the eyebox, theaspect ratios of the grating features can be varied to more evenlydistribute light within the expanded output beams and provide morebalanced illumination throughout the eyebox. Both the length l and thewidth w of the grating features can be varied to control desireddistributions of light within the eyebox. In addition to varying aspectratios in the x and y coordinate directions, the aspect ratios of thegrating features can be varied in other orientations, particularly inorientations normal to the grating vectors. In areas of the compoundgrating at which too much light is out-coupled, the aspect ratio oflocal grating features can be arranged to favor diffractive orders thattend to distribute light within the waveguide toward other areas of thecompound grating at which too little light may otherwise be out-coupled.In addition, out-coupling can be promoted is areas that would otherwiseout-couple too little light.

In addition to defining the unit cells by the grating vectors andattendant diffractive orders that the arrangement of unit cells willsupport, the shape of the grating features within the unit cells as wellas the relative refractive index and the height or depth of the gratingfeatures can be controlled to influence distributions of light among thediffractive orders of each unit cell.

While the decoded light is generally ejected from the waveguide byencounters with a grating pattern having a grating vector along thenominal direction of propagation, the three grating patterns can beoriented at 60 degrees to one another with substantially equal pitch topreserve angular encoding while providing more opportunities forejecting light from the waveguide as shown for example in FIGS. 16through 18. With the first grating pattern oriented perpendicular to thenominal direction of propagation, the second and third grating patternscan be respectively oriented at plus 60 degrees and minus 60 degreesthereto. Image-bearing beam portions encountering the second gratingpattern can be diffracted through a first order on a pathwaysubstantially perpendicular to the third grating pattern. In thisorientation, the beam portions can be ejected from the waveguide bydiffractions from the third grating pattern through a similar firstorder. Similarly, image-bearing beam portions encountering the thirdgrating pattern can be diffracted through a first order on a pathwaysubstantially perpendicular to the second grating pattern. In thisorientation, the beam portions can be ejected from the waveguide bydiffractions from the second grating pattern through a similar firstorder.

Succeeding encounters with the second grating pattern through theintermediacy of an encounter with the first grating pattern allow thesubsequent encounter with the second grating pattern to eject decodedlight from the waveguide toward the eyebox. Without the intermediacy ofan encounter with the first diffraction grating, successive encounterswith the second diffraction grating or successive encounters with thesecond diffraction grating through the first diffraction order canrestore the so diffracted beam portions along the nominal direction ofpropagation. Successive encounters through the second, first, and thirddiffraction patterns through the first order can reorient the sodiffracted portions in a direction opposite to the nominal direction ofpropagation. Combined with the zero-order diffractions from the threegrating patterns that preserve the instant direction of propagation inthe x-y plane, the various encounters and reencounters with the threegrating patterns circulate light portions of each of the image-bearingbeams throughout the out-coupling optic in ways that preserve theoriginal encoding of the image-bearing beams for ejection from thewaveguide through various encounters with each of the three gratingpatterns.

Fabrication of Grating Patterns in Image Light Guides

The compound grating patterns can be formed as a volume hologram opticalelement (HOE). As is well known to those skilled in the art, a volumeHOE is a hologram where the thickness of the recording material is muchlarger than the light wavelength used for recording. Alternatively, theoverlapping grating patterns can be realized as a 2-dimensional (2-D)photonic crystal. Alternatively, the compound grating patterns can beformed as a 3-dimensional (3-D) photonic crystal. As is known to thoseskilled in the art, 2-D photonic crystals can be fabricated byphotolithography, or by drilling holes in a suitable substrate. As isknown to those skilled in the art, fabrication methods for 3-D photoniccrystals include stacking multiple 2-D layers on top of each other,direct laser writing, or instigating self-assembly of spheres in amatrix and dissolving the spheres.

The in-coupling and out-coupling diffractive optics IDO and ODO can be,but are not limited to, diffraction gratings or formed as volumeholograms, or formed from a holographic polymer dispersed liquidcrystal, for example. The waveguide substrate S of the image light guideis typically glass or other optical material with sufficient index ofrefraction for supporting TIR transmission between in-couplingdiffractive optic, distribution gratings, and out-coupling diffractiveoptic.

The in-coupling diffractive optics IDO and out-coupling diffractiveoptics ODO can have different orientations and grating periodsappropriate to their functions. After proper surface preparation of aglass substrate blank, the diffraction components can be formed on oneor both outer surfaces of the image light guide using nano-imprintingmethods, for example. At least one of the in-couplings and out-couplingscan be a surface relief diffraction grating.

As known to those skilled in the arts, one method for the removal ofsubstrate material is by electron-beam machining. Electron-beammachining is a process where high-velocity electrons concentrated into anarrow beam are directed toward the work piece, creating heat andvaporizing the material.

FIG. 27A and FIG. 27B show processed SEM images of electron-beammanufactured grating patterns. FIG. 27A depicts a compound gratingpattern 850 as a manufactured realization of the grating pattern 660 ofFIG. 18, and FIG. 27B depicts a compound grating pattern 860 as amanufactured realization of the grating pattern shown in FIG. 19. Inboth cases, and as known to those skilled in the arts, the manufacturedresults will approximate the idealization of the grating patterns. Inparticular, the grating features 662 shown in FIG. 18 are manufacturedas grating 852 features having rounded corners as shown in FIG. 27A.Similarly, the sharp corners of the grating features 704 shown in FIG.19 are realized in manufacturing as grating features 862 having roundedcorners as shown in FIG. 27B. Different manufacturing processes willresult in different degrees of approximation of fine features of agrating pattern.

The invention has been described in detail with reference to presentlypreferred embodiments, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention. The presently disclosed embodiments are therefore consideredin all respects to be illustrative and not restrictive.

1. An image light guide for conveying a virtual image comprising: awaveguide having a transmissive substrate with front and back surfacesfor propagating image-bearing light beams along a length of thewaveguide; an in-coupling diffractive optic formed along the waveguidefor diffracting the image-bearing light beams from an image source intothe waveguide for propagating the image-bearing light beams along thelength of the waveguide in an angularly encoded form; an out-couplingdiffractive optic spaced apart from the in-coupling diffractive opticalong the length of the waveguide for diffracting the image-bearinglight beams from the waveguide in an angularly decoded form toward aneyebox for viewing the image from the image source; the out-couplingdiffractive optic including a plurality of grating patterns in differentorientations that overlap along the length of the waveguide; one or moreof the grating patterns being arranged in a primary orientation fordiffracting first portions of each of the image-bearing light beams,which propagate in an original nominal direction along the waveguide, inthe angularly decoded form toward the eyebox; and one or more other ofthe grating patterns being arranged in one or more non-primaryorientations for diffracting second portions of each of theimage-bearing light beams out of the original direction of propagationalong the waveguide and for further diffracting the second portions ofeach of the image-bearing light beams back toward the original nominaldirection of propagation along the waveguide in positions that arerelatively offset in a direction normal to the original nominaldirection of propagation.
 2. The image light guide of claim 1 in whichthe first portions of each of the image-bearing light beams arerelatively displaced in the original nominal direction of propagationalong the waveguide for expanding a first dimension of an area ofoverlap among the image-bearing light beams within the eyebox.
 3. Theimage light guide of claim 2 in which the second portions of each of theimage-bearing light beams are relatively displaced in the directionnormal to the original nominal direction of propagation along thewaveguide for expanding a second dimension of the area of overlap amongthe image-bearing light beams within the eyebox.
 4. The image lightguide of claim 1 in which the one or more of the grating patternsarranged in a primary orientation include a first grating pattern havinga first grating vector, and the one or more other of the gratingpatterns arranged in one or more of the non-primary orientations includea second grating pattern having a second grating vector that is inclinedto the first grating vector.
 5. The image light guide of claim 4 inwhich the second grating vector is inclined at an angle of at least 30degrees to the first grating vector.
 6. The image light guide of claim 4in which the one or more other of the grating patterns arranged in oneor more of the non-primary orientations include a third grating patternhaving a third grating vector that is inclined to both the first andsecond grating vectors.
 7. The image light guide of claim 6 in which thesecond and third grating vectors are inclined to the first gratingvector by approximately the same amount in different directions.
 8. Theimage light guide of claim 6 in which the second and third gratingvectors are in part defined by a second pitch and a third pitch that areequal to each other.
 9. The image light guide of claim 8 in which thefirst grating vector is in part defined by a first pitch that isdifferent from the second and third pitches.
 10. The image light guideof claim 1 in which the plurality of grating patterns is formed by anarrangement of grating features in the same plane.
 11. The image lightguide of claim 1 in which the plurality of grating patterns is formed byan arrangement of grating features in different planes.
 12. The imagelight guide of claim 1 in which the plurality of grating patterns indifferent orientations are formed in a common plane.
 13. The image lightguide of claim 1 in which the plurality of grating patterns in differentorientations are formed within a volume hologram.
 14. An image lightguide for conveying a virtual image comprising: a waveguide having atransmissive substrate with front and back surfaces for propagatingimage-bearing light beams along a length of the waveguide; anin-coupling diffractive optic formed along the waveguide for diffractingthe image-bearing light beams from an image source into the waveguidefor propagating the image-bearing light beams along the length of thewaveguide in an angularly encoded form; an out-coupling diffractiveoptic spaced apart from the in-coupling diffractive optic along thelength of the waveguide for diffracting the image-bearing light beamsfrom the waveguide in an angularly decoded form toward an eyebox forviewing the image from the image source; and the out-couplingdiffractive optic being formed as a diffraction lattice having aplurality of unit cells in a plane defining a plurality of diffractionpatterns in different orientations within the plane.
 15. The image lightguide of claim 14 in which the diffraction patterns provide forrelatively displacing portions of each of the image-bearing beams in twodimensions for expanding an area of overlap among the image-bearingbeams within the eyebox.
 16. The image light guide of claim 15 in whichthe diffraction lattice comprises a plurality of diffractive featuresarranged in a pattern and distinguished by a refractive index differencewithin the plane.
 17. The image light guide of claim 16 in which theplane includes horizontal and vertical dimensions and the diffractivefeatures have corresponding horizontal and vertical dimensions that arenot equal to regulate distributions of light within each of theimage-bearing beams.
 18. The image light guide of claim 16 in whichportions of the diffractive features overlap.
 19. The image light guideof claim 16 in which the diffractive features are in the shape ofpolygons.
 20. The image light guide of claim 16 in which the diffractivefeatures have a rounded shape.
 21. The image light guide of claim 15 inwhich the unit cells are in the shape of a polygon.
 22. The image lightguide of claim 21 in which the unit cells are in the shape of arectangle.
 23. The image light guide of claim 21 in which the unit cellsare in the shape of a hexagon.
 24. The image light guide of claim 15 inwhich one of the diffraction patterns is arranged in a primaryorientation for diffracting first portions of each of the image-bearinglight beams, which propagate in an original nominal direction along thewaveguide, in the angularly decoded form toward the eyebox, and two ormore other of the diffraction patterns are arranged in non-primaryorientations for diffracting second portions of each of theimage-bearing light beams out of the original nominal direction ofpropagation along the waveguide and for further diffracting the secondportions of each of the image-bearing light beams back toward theoriginal nominal direction of propagation along the waveguide inpositions that are relatively offset in a direction normal to theoriginal nominal direction or propagation.
 25. The image light guide ofclaim 24 in which the two or more of the other of the diffractionpatterns arranged in the non-primary orientations are oriented by equalbut opposite angles with respect to the diffraction pattern arranged ina primary orientation.
 26. An image light guide for conveying a virtualimage comprising: a waveguide having a transmissive substrate with frontand back surfaces for propagating image-bearing light beams along alength of the waveguide; an in-coupling diffractive optic formed alongthe waveguide for diffracting the image-bearing light beams from animage source into the waveguide for propagating the image-bearing lightbeams along the length of the waveguide in an angularly encoded form; anout-coupling diffractive optic spaced apart from the in-couplingdiffractive optic along the length of the waveguide for diffracting theimage-bearing light beams from the waveguide in an angularly decodedform toward an eyebox for viewing the image from the image source; theout-coupling diffractive optic being formed as a diffraction latticehaving a plurality of grating features in a plane defining a pluralityof diffraction patterns having different orientations within the plane;each of the grating features including a length and width in the planedefining an aspect ratio of length to width; and at least some of thegrating features having an aspect ratio that departs from unity foraltering diffraction efficiencies between the diffraction patterns. 27.The image light guide of claim 26 in which in which the image-bearingbeams approach the out-coupling diffractive optic along a nominaldirection of propagation, the length of the grating features extendsalong the nominal direction of propagation, the width of the gratingfeatures extends perpendicular to the direction of propagation, and thegrating features having an aspect ratio that departs from unity having alength that exceeds their width for enhancing the efficiency ofdiffractions that redirect portions of the image bearing beams indirections that depart from the nominal direction of propagation. 28.The image light guide of claim 26 in which the image-bearing beamsapproach the out-coupling diffractive optic along a nominal direction ofpropagation, and the aspect ratio of the grating features having anaspect ratio that departs from unity varies along the nominal directionof propagation.
 29. The image light guide of claim 26 in which theimage-bearing beams approach the out-coupling diffractive optic along anominal direction of propagation, and the grating features having anaspect ratio that departs from unity include grating features thatextend contiguously along the nominal direction of propagation.